A well-known closure for a metal can comprises a can end fixed to the can body by a double seam which extends radially inwardly into a chuck wall and, optionally, a countersink to a centre panel. The circumferential score around the edge of the centre panel and adjacent the countersink and/or chuck wall, dictates the removable area of the panel. The benefit of the removal of the majority of the centre panel is the ease of access to the can contents. In the case when the can contents comprise a solid or semi-solid food product, dispensing and access to the product is relatively straightforward.
A metal tab that is fixed by a rivet to the removable centre panel has a nose portion positioned above the score. When a handle of the tab is raised relative to the can end, the nose portion of the tab pierces the score and breaks or “pops” the score over an initial arc. By pushing the tab over the seam until the tab meets the peripheral chuck wall of the end, the initial arc is propagated and tears over a larger arc of the score. In a final opening stage, the tab and end panel are pulled out away from the can body and full opening is achieved as the end peels away from the can body.
Processing, handling and storage of filled metal cans often results in increased internal pressures within the cans. This can in turn give rise to so-called “peaking” effects which may deform the can closure and even possibly fracture the closure along the circumferential score. Whilst this problem may be overcome by making closures of sufficiently thick metal plate, such an approach is undesirable as it results in significantly increased production costs. The conventional approach to mitigating peaking effects is therefore to introduce beads (sometimes referred to as “furrows”) into the can closure profile to provide additional strength whilst minimising the metal plate thickness requirement.
FIG. 1 is a plan view of an easy open can end. The end includes a series of terraces 1, a circumferential score 2, a seaming panel 3, and wing-shaped beads 4 adjacent a rivet 5. The can end of the FIG. 1 design has a specific chord length along which the panel folds when the tab 6 is raised for opening. This chord is illustrated by the broken line 9 in the Figure.
The design of FIG. 1 may be improved, at least in respect of its pressure performance, by including a bead that extends around the entire circumference of the closure, and which passes between the score 2 and the nose portion 8 of the tab 6. This bead is indicated by reference numeral 7 in FIG. 1. The bead 7, and its location close to and parallel with the score 2, strengthens the closure in the region of the score 2, tending to prevent unintended fracturing of the score.
The design of the can closure illustrated in FIG. 1 has a number of disadvantages. Whilst it does achieve a satisfactory pressure performance, its opening performance is not so good primarily due to the short length of the chord 9 between the two points where it intersects with the score. This is caused by the presence of the strengthening bead 7 between the nose portion 8 and the score 2 which tends to prevent an initial fracture of the score, induced when the tab is raised, from propagating around the score to a sufficient extent.
A solution to this problem is to terminate the bead 7 on each side of the tab 6, i.e. to provide a break in the bead 7 in the region behind the tab. However, it has been found that merely terminating the bead 7 results in an increased risk of peaking along the score line in the region of the break. A further solution that has been proposed, see EP1577222, is to maintain the bead 7 as shown in FIG. 1, and introduce an additional pair of relatively short beads on either side of the rivet 5. These project outwardly from the rivet region in a generally circumferential direction. The additional beads provide a fold line about which the closure tends to fold when the tab is raised, counteracting the strengthening effect of the bead 7.
An important feature of can closures is their ability to resist abuse during transport and stacking. A particular problem in this regard is the possibility that when a filled can is stacked on top of another filled can, e.g. during transport, the base of the upper can pushes down on the tab of the lower can. This can cause the score formed around the closure of the lower can to fracture. A known solution to this problem is to form a pair of downwardly projecting points or nibs on either side of the tab and which project slightly further than the point of the tab nose. These additional points typically make contact with the surface of the closure in the unopened configuration and, in the event of an impact on the can, e.g. due to stacking, prevent the nose from coming into contact with the can closure. When the handle of the tab is raised to open the closure however, the tab tends to pivot about these points allowing the nose to impact the closure and fracture the score. It is possible to achieve a similar effect by providing a pair of raised dimples on the closure, under and in contact with the tab.
Considering again the design of FIG. 1 and other designs such as EP1577222 that provide a circumferential bead extending behind the nose of the tab, this bead will tend to interfere with the abuse protection points or dimples described in the preceding paragraph.